1. Field of the Invention
The present invention relates to an access method of an actuator used in a disk memory apparatus and so on, and a control apparatus therefor, and more particularly to an access method of the actuator for moving it from a starting position to a predetermined target position with a high speed and a control apparatus therefor.
2. Description of the Prior Art
Recently, the recording capacity of an information recording-reproducing apparatus such as a magnetic disk apparatus or an optical disk apparatus has been significantly increased. Accordingly, improvement of the access speed of a transducer to a target track of the magnetic disk or the optical disk is required in the recording or reproducing of data. A Bang-Bang driving method is known as an access control method of an actuator for positioning the transducer mounted on the actuator of the information recording-reproducing apparatus. This method is an open loop control system, and is a minimum time control method which applies an acceleration command and a deceleration command alternately to the actuator.
FIG. 13 is a diagram of an actuator driving circuit of an access control apparatus in prior art. In FIG. 13, reference numeral 101 designates an operational amplifier, and reference numeral 102 designates a power amplifier including transistors 102A and 102B. Reference numeral 103 represents a magnetic coil 103 having an inductance 104 and a resistance 105 which are connected in series. Reference numeral 106 is a current detection resistor which detects a current flowing through the magnetic coil 103. A counter electromotive voltage which is generated in the magnetic coil 103 is represented by a generator 107. A bang-bang command signal 100 of a rectangular waveform is applied to the non-invert input (+) of the operational amplifier 101. The output of the operational amplifier 101 is amplified by the power amplifier 102 and is applied to one terminal of the magnetic coil 103 of an actuator. The other terminal of the magnetic coil 103 is grounded through the current detection resistor 106.
In the driving circuit, the other terminal of the magnetic coil 103 is coupled to the invert input (-) of the operational amplifier 101, and thereby the current flowing through the magnetic coil 103 is in proportional to the input voltage at the non-invert (+) of the operational amplifier 101, and a constant current operation is realized within a linear operation range of the driving circuit. On the other hand, in the event that a sufficiently large input voltage is applied to the non-invert input (+) of the operational amplifier 101 such as the Bang-Bang signal 100 shown in FIG. 13, the operational amplifier 101 is saturated, and the driving circuit becomes an open loop state. Consequently, transistors 102A and 102B become alternately conducting states, and power source voltages +Ve and -Ve are alternately applied to the magnetic coil 103. This operation is a constant voltage operation.
It should be noted that, in the following explanation, the influences from the inductance 104 of magnetic coil 103 and from the counter electromotive voltage in the magnetic coil 103 caused by the movement of the actuator are not considered, because they are very small.
FIG. 14 is a side view of a direct current driving actuator generally called a voice coil motor. In FIG. 14, reference numeral 111 is a magnetic coil, reference numeral 112 is a magnet which provides a bias magnetic field to the magnetic coil 111, reference numerals 113 and 114 are respectively center and outer yokes which receive magnetic flux produced at the magnet and constitute a magnetic circuit. The center and outer yokes 113 and 114 are made of materials which have relatively high relative permeability such as iron. The operating principle of the actuator in the FIG. 14 is as follows. When magnetic flux caused by the magnet 112 crosses in chain-form a current which flows through the magnetic coil 111, magnetic coil 111 moves to the direction directed by an arrow in FIG. 14 in accordance with the left-hand theory by J. A. Fleming.
Scatter of dimension on processing the yokes 113, 114 and the magnet 112 or unevenness of magnetization of the magnet 112 sometimes causes dispersion of the magnetic power for moving the magnetic coil 111 depending on the position or the moving direction of the magnetic coil 111.
FIG. 15 is a distribution diagram showing an example of unevenness of the magnetic force for moving the magnetic coil 111 depending on the position of the magnetic coil 111. In FIG. 15, an abscissa is graduated by the distances from a reference point to the positions of the magnetic coil 111 which is a movable portion of the actuator. An ordinate is graduated by the force, that is, the force constant of the actuator which is produced in the actuator, when the magnetic coil 111 is disposed in respective positions, and a unit amount of current flows through the magnetic coil 111. When the magnetic coil 111 is located in the range of X where 5 mm=Xu1&lt;X&lt;Xu2=20 mm, the produced force is almost even. However, when the magnetic coil 111 is not in the above range as the magnetic coil 111 moves toward either end of the magnetic circuit, and the produced force becomes weaker. It is understood that the unevenness of the produced force occurs.
FIG. 16 (a) is a diagram representing a current I which flows through the magnetic coil 103, FIG. 16 (b) is a diagram representing the travel velocity V of the actuator and FIG. 16 (c) is a diagram representing the travel distance X of the actuator. An abscissa of each diagram is graduated by time. Referring to FIGS. 16 (a), 16 (b) and 16 (c), the dotted lines represent the operation of the actuator when it is not influenced by the unevenness of the force of the magnetic coil 111 (for example, when the actuator is accessed in the range of X where 5 mm&lt;X&lt;10 mm, as shown in FIG. 15), and the solid lines represent the operation of the actuator when it is influenced by the dispersion of the force (for example, when the actuator is accused in the range of X where 0 mm&lt;X&lt;5 mm).
When the operation of the actuator is not influenced by the unevenness of the force, the travel velocity V of the actuator is evaluated by an integral of the current I which is applied to the actuator, and the travel distance X is also evaluated by the double-integral of the current. Consequently, an acceleration time length and a deceleration time length in which the actuator is accelerated or decelerated are calculated according to the travel velocity V and the travel distance X with an acceleration being given by the amplitude of the Bang-Bang command signal 100. To the contrary, when the operation of the actuator is influenced by the unevenness of the force, it is very difficult to quickly infer the acceleration time and the deceleration time with high precision so that the travel velocity V becomes zero at a target position of the actuator. Consequently, according to the conventional access method such as Bang-Bang driving method, an access control apparatus with a high precision and a high speed have not been realized.